Polychlorinated biphenyls (PCBs) are widespread environmental contaminants. They are toxic organic chemicals. It has been found that PCBs tend to remain in the fatty tissues of an organism once entry has been gained and manufacture of PCBs has been banned in the United States. PCBs are a class of chemically inert, chlorinated hydrocarbons. PCB mixtures have had widespread commercial use as a result of their following favorable properties: high dielectric constant, high thermal and chemical stability, low vapor pressure, low water solubility, low flammability, and high miscibility with most organic solvents, polymers, and paints. Thus, they have been used primarily as, or additives to, dielectric, hydraulic, and heat transfer fluids. It is reported that over a billion pounds of PCBs have been manufactured in the United States, and it is estimated that 300 million pounds of these are in chemical landfills and about 750 million pounds are still in use. PCBs have been found in environmental and biological samples in many locations. They tend to accumulate in sediments, soil, and biota. The widespread distribution of PCBs has been attributed to their volatilization and atmospheric transport followed by wet or dry deposition. Although PCBs are suspected carcinogen, their acute toxicity is considered to be non-toxic to slightly toxic by the LD50 method. A more significant health impact has been linked to the incomplete combustion of PCBs which form polychlorinated dibenzodioxins (PCDDs) or polychlorinated dibenzofurans (PCDFs).
The problems associated with PCB contamination in New Bedford, Mass. (EPA Region I), the Hudson River in New York (EPA Region II), and in Waukegan, Ill. (EPA Region V) are highly publicized to be among the worst in the United States in terms of concentration and total quantity of PCBs.
The PCB contamination problems pose threats to both drinking water and the fishing industry. There are also many industrial lagoons contaminated with PCBs. At present, the only proven and effective technology to treat PCB-contaminated soil is excavation followed by incineration. However, incineration is very expensive and involves costly transportation. Disposal in a secure landfill is an option; however, it has legal restrictions and liability due to mobility of contaminants after landfilling. Permitting of landfills for hazardous materials disposal is becoming increasingly expensive and difficult. Biodegradation of PCBs is an option; however, these reactions are generally slow and not technically well proven.
PCB decontamination of PCB-contaminated liquids has been widely studied; however, little work has been done in the area of PCB-contaminated sediments and sludge. Often the soils contaminated with PCB's are in a clayey state such that before decontamination by any method is possible, the contaminated soil which is highly agglomerated requires extensive pretreatment. The pretreatment of the soil often required mechanical devices and additional time and labor which added to the expense due to the difficulties in handling the contaminated soils.
Rogers, et al describes in "Mobile KPEG Destruction Unit for PCBs, Dioxins, and Furans in Contaminated Waste," a paper presented at the 13th Annual Research Symposium on Land Hazardous Waste, Cincinnati, Ohio, May 6-8, 1987, a chemical process which reduces the toxicity of PCBs in soil by removing chlorine atoms in the presence of heat with an alkali metal polyethylene glycolate reagent (APEG; e.g., NaPEG or KPEG). The mechanism for the process is as follows: an alkali metal hydroxide such as potassium hydroxide is reacted with an alcohol such as polyethylene glycol having a molecular weight of about 400 to form an alkoxide. The alkoxide reacts with a chlorine atom from PCB to produce an alkali metal salt and an ether. Toxicity studies on the reaction products, such as the AMES test for mutagenicity and bioaccumulations, have produced negative results meaning that the products are not carcinogenic and do not accumulate in the food chain. The shortcoming of the chemical treatment using KPEG is that the process is water sensitive and requires large amounts of reagent, particularly on wet soil. The heat requirement to remove substantial amounts of water from soil will be tremendous.
Physical solidification with cement, lime, and fly ash is a technique designed to prevent PCBs from leaching from waste material. Solidification binds the waste material containing PCBs mechanically with cement, lime, and fly ash into a solid that does not readily release the contaminants upon exposure to air or water. Waste material is mixed with the solidification agents and poured into cubical compartments and allowed to cure. The solidified waste is then placed into a landfill.
Reynolds describes in "Unit Operations and Processes in Environmental Engineering," Wadsworth, Inc., Belmont, Calif., 1982, a biological process called activated sludge treatment. The treatment consists of a biological reactor containing microorganisms under aerobic conditions to oxidize organic contaminants to carbon dioxide, water, and microorganism cell mass. Kane and Metha have shown in their paper "Cleanup and Closure of a PCB Contaminated Pond" that the existence of PCB cogeners appear to enhance biodegradation reactions.
Sworzyn and Ackerman accomplish oxidation of sludge to alcohols, aldehydes, and acids using a process called catalyzed wet air oxidation as described in "Interim Guidelines for the Disposal/Destruction of PCBs and PCB Items by Non-Thermal Methods," EPA 68-02-3174, U.S. Environmental Protection Agency, Washington, D.C., 1981. In this process high temperatures (320.degree. F. to 644.degree. F.) and elevated pressures (451 psi to 2503 psi) are used to oxidize sludge to alcohols, aldehydes, and acids. At higher temperatures further oxidation of the organic compounds to carbon dioxide and water is reported. Reports also indicate that in the presence of oxygen in an acidic aqueous medium at high temperatures, PCBs can also be destroyed. The end products include carbon dioxide, nitrogen gas, water vapor, volatile organics, and inorganic solids.
In a different process called soil vitrification or glassification, electric current is used to melt the soil in place. An electric current is sent through electrodes placed in the ground to the desired depth. This causes the soil to heat up to 3600.degree. F., which destroys the organic constituents in the soil including PCBs. Gases, including carbon dioxide and water vapor are collected and treated in a specially designed hood. As the crystalline material cools after treatment, it capsulates the inorganic soil components into a solid mass resembling natural obsidian. This process is described by Fitzpatrick, et al in their paper "In-Situ Vitrification--A Candidate Process for In-Situ Destruction of Hazardous Waste," presented at 7th Superfund Conference, Washington, D.C. 1986.
One of the most effective methods of disposal for PCB-contaminated soil is incineration. Incinerators usually have a primary combustion chamber which can handle solid wastes, pumpable liquid wastes, slurries, and sludge and a secondary combustion chamber which can handle only pumpable liquid or slurry wastes. The temperature at the combustion chamber varies from 2192.degree. F. (1200.degree. C.), with a 2-second detention time and 3% excess oxygen, to 2912.degree. F. (1600.degree. C.), with a 1.5-second detention time and 2% excess oxygen. Incinerators usually operate at greater than 99.9% efficiencies. Emission gases are generally monitored for oxygen, carbon monoxide, carbon dioxide, nitrogen oxides, hydrochloric acid, chlorinated organic compounds, PCBs, and particulate.
Possible incineration technologies include: liquid injection, fluidized bed, circulating bed, rotary kiln, electric infrared, electromelt, plasma arc, and Molten salt. Rotary kilns are the primary incineration technology available for treatment of wastes. Solid wastes are fed into one end of a rotating kiln and incinerated.
An EPA report entitled "PCB sediment Decontamination--Technical/Economic Assessment of Selected Alternative Treatments," PB87--133112, (EPA/600/2-86/112) describes the following summary of processes based on physical technologies (Section 5.2):
(a) Hancher, et al describes a soil-washing process which uses a kerosene-water mixture as a solvent to extract PCBs and other contaminated oil from soil. Kerosene solubilizes PCBs and oils, and water helps break up the soil particles. The mixture ratio that they found to give the most complete extraction of PCBs from contaminated soil was one part of soil to three parts of kerosene and three parts of water. They ran pilot plant experiments which showed 85% removal of PCBs from soil. PA1 (b) Adams, et al showed that the rate of chemical destruction of PCBs in soil by APEG reagents could be enhanced by adding dimethyl sulfoxide (DMSO) or ethylene diamine to the hydroxide/PEG phase. The time required to reach less than 2 ppm PCBs in the product was shown to be reduced by an order of magnitude when DMSO Was used. PA1 (c) Weitzman reported that Freon-type solvents could be used in repeated washings of PCB-contaminated soil to remove PCB to less than 2 ppm. PCB loadings to 1983 ppm were leached in an agitated extractor. Soil types used were sand-clay mixtures and a dark loam. PA1 (d) A process used by O. H. Materials uses methanol to extract PCBs from pre-dried contaminated soil. The soil is reduced to less than 25 ppm PCBs and land-farmed. Further reduction in PCB concentration could be achieved using more stages of extraction. The PCBs in the extract are concentrated by absorption on activated carbon, and the spent carbon is incinerated. PA1 (e) Scholtz and Milanowski studied the extraction of PCBs from organic and inorganic soil using water that contained 1% Tween. The overall PCB removal was less than 50%. Results showed that the additive improved removal from inorganic soil, but appeared to inhibit removal from organic soil.
Warner et. al. studied extraction of waste matrices with methylene chloride under neutral conditions with anhydrous sodium sulfate to remove any water present. This extraction method was shown to be suitable for the extraction of phenols, anilines, and neutral compounds and is disclosed in "Hazardous and Industrial Solid Waste Testing", ASTM Special Technical Publication, #805, p. 203, 1983.
The Environmental Protection Agency (EPA) initiated a research program in early 1987 which studied soil washing for removal of contaminants such as organic, volatile, semi-volatile, and non-volatile compounds using water, water containing surfactant, and water containing chealating agents. Bench scale experiments were conducted in which the wash solution was mixed with the contaminated soil in a shaker and the mixture agitated at ambient temperature. The surfactant they used was "TIDE" (an anionic surfactant manufactured by Proctor and Gamble). The agitation was carried out for 30 minutes. The contaminated soil was comprised of different size fractions (e.g. clay, silt and sand). With soil particles smaller than 250 mm, the surfactant/water wash reduced semi-volatile organic compounds contained in the soil by 43.2 %. PCB's were not included in this study.
Moses, et. al. in a paper "Use of Liquified Gas Solvent Extraction in Hazardous Waste Site Closures" prepared for presentation at AIChE 1988 Summer National Meeting, Denver, Colo. August 21-24; Paper No. 55d, describes a proprietary, patented soil/sludge/wastewater extraction process which comprises using liquified gases and critical liquids as solvent to extract contaminants from waste streams. A critical fluid is a fluid which is at thermodynamically critical pressure and temperature, meaning that there is no distinction between a liquid or a gas under these conditions. The inventors claim that these fluids under critical conditions exhibit high solubilities for organic contaminants similar to their liquid counterparts. The advantage of using a fluid at its critical temperature and pressure is significant change in its kinetic behavior, such as high rates of diffusion and low viscosity, surface tension and density. Changing pressure and temperature generally changes the solubility of various contaminants in these fluids substantially. The process described by the inventors consists of an extraction column where the solvent is contacted countercurrently with the solids containing the contaminants. The solvent exiting the extraction column carrying the contaminants flows into a solvent recovery system where clean solvent is separated from the contaminants and is returned to the extraction column. The contaminants are properly disposed of. Classes of organic compounds extracted by the process are alcohols, aldehydes, aromatics, carboxylic acids, chlorinated hydrocarbons, esters, ethers, ketones, and nitriles.
Lanny D. Weimer describes a soil/sludge washing process called Basic Extractive Sludge Treatment. The process uses an aliphatic amine solvent such as triethylamine (TEA) to separate oily sludge into solid, contaminated-free water, and oil containing contaminants. The oil containing the contaminants is usually disposed of by incineration or chemical processes. Triethylamine has a property defined as inverse miscibility. This means that below 65.degree. F. TEA is miscible with water, and above 65.degree. F., TEA is immiscible with water. Therefore, simply heating a TEA/water/oil sample above 65.degree. F. results in separation of water from oil. The process consists of a cold section and a hot section. In the cold section, the solvent is contacted with the contaminated oily sludge at 65.degree. F., whereby, the contaminant, water, and oil all dissolve in the solvent. The solvent is then separated from the decontaminated soil or sludge. The solvent flows into the hot section of the process where it is heated to 135.degree. F. at which point a phase separation occurs whereby water is separated from the oil containing the contaminants. The process may effectively separate oil and sludge, however, it is a complicated process. It requires refrigeration in the cold section of the process. It also introduces water into the contamination cycle from which the water then must be removed and decontaminated.
The processes described above all have shortcomings, not the least of which is the inability or inoperability to cope with an agglomerated or agglutinated PCB-contaminated waterwet soil. Incinerators are effective but the cost is enormously high. PCB landfilling is receiving more restrictions from the government. Chemical treatments are either water sensitive or consume too much reagents and are not yet technically proven.
Of the processes described above which may be considered to use solvents, there are two which have substantial shortcomings due to the presence of water with the mixture being cleaned or treated. The use of a solvent which is soluble in water such as methanol results in a liquid mixture where the contaminant, water and solvent are not easily separated. Furthermore, the solvent may not be easily recovered for reuse or a water effluent free of contaminants may not be easily obtained. The use of water as a solvent has the same shortcomings. When a surfactant is added to the water, the problems are increased since the surfactant acts to emulsify the contaminants so as to change a contaminant which is completely insoluble in water to a mixture containing the contaminant which is soluble to some degree in the water solvent. However, separating the contaminant and surfactant mixture from the water is very difficult, and especially obtaining a contaminant free water effluent.
The present invention overcomes the difficulties of these processes by using a solvent which is sparingly soluble in water providing an easy separation of the water and solvent and a comminuting surfactant which disperses agglomerated solid, contaminant mixtures which are wet with water and enables the easy separation of the solids from the contaminated mixture being cleaned or treated.